How Stars Form Amid Black Hole Chaos

A computer simulation shows the evolution of a molecular cloud with a solar mass of 10^4 falling toward a supermassive black hole with a solar mass of 10^6. Here, the cloud is within 0.5 parsecs of the black hole. Color scale is from 0.1 g cm^-2 to 1000 g cm^-2. A small population of stars is being ejected from the system in the this image.Credit: Science/AAAS

Deep in the center of our galaxy, circling suspiciously
close to the giant black hole lurking there, is a group of massive stars.

Now scientists have designed a model that shows for the
first time how these stars might have formed in such an extreme environment.

Astronomers have long puzzled how these massive stars came
to be in the vicinity of a huge
black hole. They couldn't have formed as most stars do, from a tenuous
cloud of gas, because this cloud would have been ripped apart by the savage
gravitational forces from the black hole nearby.

One guess was that the stars originally formed elsewhere as a cluster
and later spiraled inward. But no trace has been found of the trail of stars this
process would have left behind.

A new explanation

Now Ian Bonnell, an astronomer at the University of St
Andrews in Scotland, and William Ken Rice of the University of Edinburgh have created
a computer simulation that offers a possible explanation for how the stars
could have formed. They detailed their model in the Aug. 22 issue of the
journal Science.

In their model, a giant gas cloud plunges in toward the
black hole. As it gets close, much of the cloud is ripped apart by the black
hole's gravity, though some portion of it survives because of the turbulence of
the gas in the cloud.

This remnant forms an oval disk of gas orbiting around the
black hole, gravitationally bound but beyond the range within which it would be
sucked in. Variations in the density of the material in the disk then cause it
to condense into stars and break up, leaving the stars in an oval orbit around
the black hole where the disk used to be.

?"We've been trying to see how we could bridge the gap
between normal star formation and star formation in this kind of
environment," Bonnell told SPACE.com. "The mechanism we present
should be able to provide a variety of possible properties which will
definitely encompass the ones we observe in our galaxy."

Proof?

Though the researchers found a way that the stars near our
galaxy's central black hole could have formed, the simulation is not
proof that this is what happened.

"As satisfying as the new results are, the case for
disk fragmentation as the origin for the disk stars remains unproven,"
wrote astronomer Philip Armitage of University of Colorado, Boulder, in an
accompanying commentary article in the same issue of Science. "We
do not know whether the initial conditions assumed by Bonnell and Rice are
realized in the galactic center."

Bonnell agreed that the next step would be to try to determine
how probable their proposed scenario is compared to other suggestions for how
the stars formed.

"What we need to do next is to worry about the
likelihood of these other events happening," Bonnell said. "For the
process we've outlined, the probability is high enough that it has a real
chance of happening. I don't think it's anything that will prove to be that
controversial in the long run. I have a feeling it seems quite likely."

A widespread occurrence

The scientists are also interested in whether this process
may have occurred in other galaxies. It is difficult to study the central
regions of other galaxies because they are so far away, but it could be that
this type of star formation is widespread in the universe, Bonnell said.

"Supermassive black holes like the one in our galactic
center are fairly common in galaxies," he said, "though our black
hole is not as big as the black holes in other galaxies, which could be
thousands of times more massive."